Thule (/ˈθjuːliː/[1] Ancient Greek: Θούλη, romanized: Thúlē ; Latin: Thūlē also spelled as Thylē [2]) is the most northerly location mentioned in ancient Greek and Roman literature and cartography. First written of by the Greek explorer Pytheas of Massalia (modern-day Marseille, France) in about 320 BC, it was often described by later writers as an island north of Ireland or Britain, despite Pytheas never explicitly describing it as an island.[3] Modern interpretations have included Orkney, Shetland, Northern Scotland, the Faroe Islands, and Iceland. Other potential locations are the island of Saaremaa (Ösel) in Estonia,[4][5] or the Norwegian island of Smøla.[6]

In classical and medieval literature, ultima Thule (Latin "farthest Thule") acquired a metaphorical meaning of any distant place located beyond the "borders of the known world".[7] By the Late Middle Ages and the early modern period, the Greco-Roman Thule was often identified with the real Iceland or Greenland. Sometimes Ultima Thule was a Latin name for Greenland, when Thule was used for Iceland. By the 19th century, however, Thule was frequently identified with Norway, Denmark, the whole of Scandinavia, one of the larger Scottish islands, the Faroes, or several of those locations.[8][9]

Thule formerly gave its name to real places. In 1910, the explorer Knud Rasmussen established a missionary and trading post in north-western Greenland, which he named "Thule". It later gave its name to the northernmost United States Air Force base, Thule Air Base, in northwest Greenland. With the transfer of the base to the United States Space Force, its name was changed to Pituffik Space Base on April 6, 2023.